Make sure *err and *err_info are set when returning PCAPNG_BLOCK_NOT_SHB.

Clean up a comment and indentation while we're at it.

Change-Id: Ia2b0a3f642849dcd464e04cdca13ff05c2fbe2e6
Reviewed-on: https://code.wireshark.org/review/4717
Reviewed-by: Guy Harris <guy@alum.mit.edu>